The present invention relates to self-clinching or clinch type fasteners, and more particularly concerns an improved form of such a fastener employing novel projections to assure a predicted and predetermined locking action with a relatively thin sheet material workpiece.
Clinch type or self-clinching fasteners are well-known in the art and employ a number of clinching or staking structures and methods. These prior art fasteners are primarily of the type which are assembled in a pre-punched hole or aperture in a work piece comprising sheet material. Thereupon, the head portion of these fasteners is seated and embedded in the facing surface of the sheet material to cause some displacement of the sheet material. This displacement results in some cold flow of amterial about the surfaces of the fastener, which are formed in a fashion so as to receive and interlock with this material of the sheet or workpiece. The mechanical interlocking obtained provides a means for maintaining the fastener in a fixed and non-rotatable engagement with the sheet material, for example, to receive a mating fastener.
One particularly advantageous type of prior art clinch type fastener is shown in U.S. Pat. No. 3,967,669 to which reference is invited. Clinch type fasteners made in accordance with this patent advantageously utilizes an axially leading extruding portion which serves to size or rework a pre-existing or pre-punched aperture in the sheet of material. Accordingly, an axially following locking groove is thereby placed within an aperture of predicted size, having been preformed by the leading extruding portion. As an additional advantage, the fastener of this patent provides radially disposed projections extending axially from the head and toward the groove, these projections being sized fo displacing a volume of material of the sheet into the groove. These radial projections also serve to prevent rotation of the fastener relative to the sheet material, once embedded therein.
In fabricating the above-discussed fastener, difficulty has been encountered in attaining the desired dimensional configuration for the radial projections. The problem has become especially acute with respect to relatively small fasteners. In this regard, there is some "trade-off" which occurs between the radial extent of the projections, for preventing relative rotation, and the overall volume of the projections, which corresponds to the volume of material displaced thereby into the groove. That is, when working with relatively thin sheet material, only a relatively small amount of material is to be displaced into the groove. However, in the cold forming of the projections it is difficult to displace sufficient material to fill out the desired projection profile. As such, projections result which have an insufficient radial extent to provide the desired degree of resistance to torque or rotation relative to the workpiece or sheet of material once the fastener is embedded therein. On the other hand, increasing the size of the radial projections tends to result in too great a volume being displaced upon seating of the fastener, such that the volume of material displaced thereby tends to overfill the groove. Such overfilling is undesirable, in that it is unsightly and leaves a rough exterior that may require machining.
More specifically, with the present invention the exposed undersurface of the projections or lobes is relieved. Thus, upon cold forming of the metal during fabrication of the projections it is assured that material will be displaced or forced radially outward to form the desired lobe profile or configuration. Also, the relieved configuration of the lobes are such that when embedded in a workpiece or section of sheet material additional edge surfaces are available to resist rotation of the fastener.